Change History (3)

Usernames need to be A-Z and 0-9 plus a couple other standard characters I think like - and _. This is for code and other reasons to ensure one can login no matter the charset and things like that (stubs too).

Display names exist so that you can show users in a different way with whatever characters you want.

However, the change profile page probably shouldn't check the username since the field is disabled and for backwards compatibility such as this.